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58 lines
2 KiB
Text
58 lines
2 KiB
Text
Preroll
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-------
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A sink element can only complete the state change to PAUSED after a buffer
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has been queued on the input pad or pads. This process is called prerolling
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and is needed to fill the pipeline with buffers so that the transition to
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PLAYING goes as fast as possible with no visual delay for the user.
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Preroll is also crucial in maintaining correct audio and video synchronisation
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and ensuring that no buffers are dropped in the sinks.
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After receiving a buffer (or EOS) on a pad the chain/event function should
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wait to render the buffers or in the EOS case, wait to post the EOS
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message. While waiting, the sink will wait for the preroll cond to be signalled.
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Several things can happen that require the preroll cond to be signalled. This
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include state changes or flush events. The prerolling is implemented in
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sinks (see part-element-sink.txt)
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Committing the state
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~~~~~~~~~~~~~~~~~~~~
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When going to PAUSED and PLAYING a buffer should be queued in the pad. We also
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make this requirement for going to PLAYING since a flush event in the PAUSED
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state could unqueue the buffer again.
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The state is commited in the following conditions:
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- a buffer is received on a sinkpad
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- an EOS is received on a sinkpad.
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We require the state change to be commited in EOS as well since an EOS means
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by definition that no buffer is going to arrive anymore.
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After the state is commited, a blocking wait should be performed for the
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next event. Some sinks might render the preroll buffer before starting this
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blocking wait.
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Unlocking the preroll
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~~~~~~~~~~~~~~~~~~~~~
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The following conditions unlock the preroll:
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- a state change
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- a flush event
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When the preroll is unlocked by a flush event, a return value of
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GST_FLOW_WRONG_STATE is to be returned to the peer pad.
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When preroll is unlocked by a state change to PLAYING, playback and
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rendering of the buffers shall start.
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When preroll is unlocked by a state change to READY, the buffer is
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to be discarded and a GST_FLOW_WRONG_STATE shall be returned to the
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peer element.
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